Retroviruses are a type of RNA virus that replicate through a DNA intermediate. FIG. 1 illustrates examples of two types of retroviruses--simpler "S Type" and more complex "MC Type". All retroviruses have gag, pol, and env genes. For some retroviruses (e.g. spleen necrosis virus; murine leukemia virus) only these genes are needed for viral replication. Such viruses are called "simpler" or "S Type" retroviruses. (See e.g. U.S. Pat. Nos. 4,650,764; 4,980,289; and 5,124,263. The disclosure of these patents and of all other publications referred to herein are incorporated by reference as if fully set forth herein.) Other retroviruses, called "more complex" or "MC Type" retroviruses, need additional genes for replication. Among the more complex retroviruses are human immunodeficiency virus (HIV), human spumaretrovirus, human T-lymphotropic virus type I (HTLV-I), and bovine leukemia virus (BLV).
The additional genes of the more complex retroviruses are thought essential for replication of the natural virus. In this regard, the additional genes in complex retroviruses are known to code for proteins that act on transcription, splicing, and polyadenylation. See generally H. Temin, et. al., The Retroviridae, v. 1, New York, 1, 5 (1992).
The genomes of both simpler and more complex retroviruses have some common features. Both types of RNA viruses replicate through a DNA intermediate. Therefore, both simpler and more complex retroviruses have DNA and RNA genomes. The viral DNA genomes for both types of retroviruses are bounded by long terminal repeats (LTRs). These LTRs contain enhancer, promoter, usually 3' RNA processing sequences, and integration sequences ("att").
Simpler and more complex retroviruses have different infection cycles. Temin (The Retroviridae, Supra pp. 1, 6-7) describes these two different infection cycles. The primary difference relates to the involvement of regulatory proteins in the more complex retrovirus cycle.
In nature, disease caused by simpler retroviruses are found in various non-mammalian hosts, but not primates or ungulates. However, diseases caused by more complex retroviruses are prevalent in ungulates and primates (especially humans).
For many more complex retroviruses there is, as yet, no safe and effective vaccine against the disease caused by the virus. Reasons for this are believed to be that certain more complex regulatory retroviral proteins interfere with the immune response and/or that retroviruses tend to mutate too rapidly in vivo for the body to provide a long-term immune response. Therefore, there is a need to find a way to produce representative spectrums of varied more complex retroviral Gag, Pol, and Env antigens, albeit without producing other disease-causing complex retroviral proteins.